Alarm system

ABSTRACT

This invention relates generally to alarm systems and, more particularly, to an alarm system using a two-way communication link between a monitoring station and a transponder station at a remote location the alarm status of which is being monitored, such system employing encoding techniques based on the generation of a truly random signal at the monitoring station.

ALARM v 23 COMPARATOR/ DECODER ALARM OUTPUT 9 I0 DISPLAY SYSTEMS IUnited States Patent 1 [1 11 3,

Getchell [4 Jan. 23, 1973 [5 ALARM SYSTEM 3,145,380 r, 8/1964 Currie ..340/-l52 T 3,587,051 6/1971 Hovey.... ..340/l64 R [75] Invent l' Gmhd"carlsle Mass 3,678,512 7 1972 Fergus ...340/40a [73] Assignee:Signatron, lnc., Lexington, Mass.

Primary Examiner-Thomas B. Habecker [22] Flled: 1972 Attorney-Robert F.OConnell et al. [2]] Appl. No.: 218,360

[57] ABSTRACT [52] CL 340/498, 340/152 T 340/164 R This inventionrelates generally to alarm systems and, 51 Int. Cl. .Q. "Gosh 26/00Particularly System "Sing a wmway 58 Field of Search ..340/l64 R -152'1' 40s link between a and a transponder station at a remote locationthe alarm [56] References cued status of which is being monitored, suchsystem employing encoding techniquesbased on the generation UNITEDSTATES PATENTS of a truly random signal at the monitoring station.

3,021,398 2/1962 Barnett ..340/408 25 Claims, 24 Drawing Figures 20 zszgifg TRANSMITTER DATA RECEIVER A INPUT SIGNAL l i i l TRANSPONDER CODETRANSMISSION ENQODER PROGRAM CODE LINK PROGRAM s in TRANQMTTER Pmmmmzsma 3.713 142 sum 03 or 15 f =f IAHI) GATED RANDOM BIT STORAGE STEAMPATENIEB JAN 23 I975 ,713,142 SHEET OR 0F 15 /5| s I RECEIVED l POWER 4I TRANSMISSION v DATA I SUPPLY R ls POWER LINK 4- AMPLI- F a l FIER --5450 l l L 63 FILTER H I I r 52 VOLT' 62 I 53 I Com REFERENCE (I I I I l II I I POWER.

I J SUPPLY TRANS I e| l LINE RECE'VED I 64 LOW PASS r DATA I FILTER 6 IPATENTEUmzs I975 3.713. 142

SHEET 05 0F 15 66 66 as RANDOM INPUT DATA 1 1 Q A B B, B B2 B3 B3 84 5'V T TT ALARM ALARM 2- CODE CONTROL PROGRAM ALARM -o .S/AAAA A5 4 A M) PP2 P3 P4 l 0 5 Ba PAIENIEDJAII 23 I975 3,713. 142 SHEET 08 [1F 15OUTPUTS VARIABLE I I v INPUTS A B O D E 0 O o I I 0 I o I I o I 0 I o 0l O PAIENIEDJAII 23 I973 .713.142 SHEEI UBUF 15 L To TRANSMITTER RZSSSMDATA- S'GNA sToRAGE L UNIT GENERAToR B, B VB 7 B4,

SEQUENTIALLY CODE N BIT APPLIED SIMULATED I PLUG TRANsMITTED ALARMsIGNALs MESSAGE ENCODER l4 I I l l COMPARATOR 23 72 DEcoDER I I I I 2 NI ALL zERos I II 5* a; BIL 8* ALL oNEs ALL ZEROS l 2 a 4 DETEcToR OR ALLoNEs DETECTOR DATA FROM STORAGE REcEIvER 32 73 I A LoGIc STROBE T LFsIGNAL ALARM STROBE Bus GENERATOR A 7| GATE T. L. I=.

1 STORE sToRE I I I I I TORE STORE STORE sToRE ALL A A2 A5 76 CLEAR 7O7O 75 v V TROUBLE NORMAL A A AZNI LINE L FAULT j T0 ALAHM NH IN INH &DISPLAY CIRCUITS PAIENIEnJIIIIzs I973 3.713.142

SHEET 08 [IF 15 TO OTHER ALARM OUTPUT AUDIO AND DISPLAY SUBSYSTEMSCIRCUIT LATCH ALARM INTE- ING AND vIsUAL STORAGE GRATOR CIRCUIT I GATEDISPLAY 7 0 86 l a; 82 FROM COMPARATOR BLINK SIGNAL DECODER 23 r MANUALI RESET ALARM M STORE STROBE FIG-9 I I I I' I BITTIME ITBITSTIOITOI I Il I I I ALARM DATA ILI I I I l I I I ALARM STORAGE N0 ALARM cowlTIoNlALARM CONDITION OUTPUT I I I I I l I I ALARM STROBE I I I I SIGNAL L I lI In I To OTHER ALARM OUTPUT CIRCUITS TO AUDIO A cIRcUIT V FROM 88 oooMPARAToR PUP \I coMRAR- 94 DECODER 23 FLOP ATOR NAND 8| 92 AND 93 .Vth85 ALARM I 9 STROBE SIGNAL I FIGQB MANUAL BLINK RESET SIGNALPATENIEnJmamTs 3,713,142

SHEET 1UOF 15 29 TIMING CIRCUIT I 97 REsET 99 I I SYNC CLOCK 98 TIMINGCHAIN i l DETECTOR osc CIRCUIT T T I 1 STROBE l- PULSE l m TRANS- iLM'SSION. 1L SIGNAL DATA I3 l DATA RECEIVE f I77 CODE CONTROL iTRANSPONDER PROGRAM ENCODER Q T I ALARM LINE LATCHING ENCODER GATETRANSMITTER CIRCUIT I .00 1 Km,

MANUAL REsET t OUTPUT DATA sToRE sl ALARM INPUT FROM SENSORS PATENTEDJAH23 I975 3.713. 142 SHEET 11 [1F 15 v 95 i TO SYNC DETECTOR TRANSMISSIONNOISE LINE l3 FILTER TO RECEIVER DATA STORAGE I02 28 REF 0 V| (1) VI,MAX

TIME

'PAIiIIIinIIIIza I975 3.713; 142

SHEET 12DF 15 I TRANSPONDER CLOCK I TO ZERO MONSTABLE I MULTI- ,-I

I I I I osc COUNT DOWN TIMING GENERATOR I l 98 lr-I 99 1d I DECODER d II 29 J I VIBRATOR I I 97-I I I SCHIMITT FROM TRIGGER I RECEIVER 22 ICIRCUIT I SYNC. DETECTOR I FIGIZ |I'MESSAGE FRAME I I I,SYNC|4 DATAINPUT I1DATA OUTPUT-PI I I T :TI ITZ T3 T4 IT I T r T8 I T IT TRECElVERi I I I loklfI I I I I I STROBE I -I "II OUTPUT I I I I I FROM IMULT IVIBRATOR l I I I I I I I osc OFF AND I l I I I I I I ENABLED I l ll @535; I I I I I I I i fi' I I l I I I I I L l I09 I I I I I I I I IPATEN'IEDJIIIIZB I975 3,713,142 SHEET 13 0F 15 /98 I osc FLIP-FLOP COUNTDOWN CIRCUITS/HO II I I TIMING DECODER PULSE SHAPER RESET TO ZERO TOMULTIVIBRATOR FROM SYNC.

V DETECTOR Id I8 I FIGI?) I I "1 II II II II II II II II II I I I I I II I 1e I I l I II I I I I I I I I I -I I I I I n IL n n n n n n II fl II I I I I I l l I I I l I I I I I I I I PAIEIIIEIIJIm I975 3.713. 142

' SHEET 114 0F 15 STORE DATA TIMING T INPUT DATA FROM INPUT DATA sToRETRANSPONDER SHIFT REGISTER RECEIVER CODE I J30 CONTROL l N D R PROGRAM 1E Co E I H 1 H3 J H, x -32 I ENCODER GATE ENCODE TIMING REsET LATCH l li i II4 ALARMS OUTPUT DATA STORE oNE SHIFT REGISTER TRANSMIT TIMINGOUTPUT DATA TO TRANSMITTER 2| FIG.I4

PATEN Enmzama 3.713.142

SHEET lSUF 15 22 [SIGNAL LINE RECEIVER T ONDER f INPUT DATA T RANSMISSION LIN E ;Q= TRANSPONDER OUTPUT DATA TRANSMISSION Ll NE --'W\,OUTPUT DATA FIG. [5A

ALARM SYSTEM BACKGROUND OF THE INVENTION Alarm system installations,particularly those which are used to protect relatively large industrialinstitutions having a highly valued inventory such as money or othervaluable materials, as in banks, industrial plants, governmentinstallations housing classified documents, and the like, should be suchthat it is substantially impossible for an intruder to thwart theoperation of such a system and, thus, enter the premises undetected.

In simple alarm systems in present use, intruders often are able to takeappropriate action to prevent the annunciation of an alarm. Even in themore complicated of such systems using selective coding andcommunication techniques for informing a central monitor station of thealarm status of a station at a remote location, a sophisticated intrudermay be able, for example, to break the system code using appropriateelectronic techniques and thereby produce a simulated signal indicatingan all clear status for insertion into the communication link betweenthe stations and, accordingly, avoid detection.

Further, it is desirable that an alarm be able to detect and displayother undesirable conditions which may exist in the system, as, forexample, the presence of an open or short circuit in the communicationlink, or of other troubles which may arise in the system, such asmalfunctions in the e'quipment being used or tempering with the systemby an intruder.

DESCRIPTION OF THE PRIOR ART Alarm systems of the prior art forinstallations which require a relatively high degree of protectionusually utilize a one-way communication link for transmitting alarminformation from a remote location to a central monitoring station. Insuch systems, for example, an appropriately encoded signal whichcontains the alarm status information is entirely generated at theremote location and is transmitted to the central monitor station whereit is decoded and the alarm status information displayed in someappropriate audible or visual manner. Such systems may utilize eithersignals of a non-random nature which are suitably encoded at the remotestation with the alarm status information, or, alternatively, they mayutilize signals of a psuedo-random nature for encoding. Thecomplementary decoding process utilized at the monitor station detectsand identifies the alarm status information which is carried by theremotely generated signals.

Such systems, however, can often be defeated by an intruder throughappropriate electronic techniques wherein the intruder, for example, canuse his own equipment to produce a simulated signal corresponding to thecoded signal which signifies a normal or allclear condition and caninsert such artificially produced signal onto the communications link inplace of the signal generated by the system itself. In that way themonitor station is not alerted to the presence of the intruder and thetrue nature of the alarm status at the remote location remainsundetected.

Thus, where the encoded signal is generated at a remote station, whetherusing a non-random signal or a pseudo-random signal in the encodingprocess, an intruder normally need only record the encoded signal in theall clear state and, by suitable statistical analysis thereof, fashion acorrectly synchronized simulated signal having the same characteristicsas the true encoded signal, without the need for a knowledge of thespecific coding scheme which is being used therein.

Even in systems where the code scheme being used can be changedperiodically, the number of different code schemes available is usuallyquite limited in the prior art systems and the capability of an intruderto reproduce the desired simulated all clear signal is still relativelyhigh.

While more elaborate coding systems may be devised by the prior art todefeat intruders in certain applications, such as for extremelysensitive military security purposes where cost is often no object, themore complex equipment required for such purposes increases the coststhereof to a point where the use of such elaborate schemes in theapplications in which the system of the invention is intended becomesprohibitive.

As discussed more fully below, the system of the invention is designedso that its costs are substantially the same as the costs of prior artsystems for use in the same applications and are well below the cost ofthe more complex coding systems used in other highly sensitive securityapplications. However, despite the relatively low cost of the system ofthe invention, the system is much less susceptible to the recording andanalysis techniques discussed above and the possibility that an intrudermight defeat the system is considerably reduced in comparison with suchpossibility in connection with presently known systems of comparablecost.

DESCRIPTION OF THE INVENTION The above discussed disadvantages of priorart systems are overcome by the system of the invention, which permitsthe use of an extremely large number of different code schemes for usetherein. While the techniques used in the invention are somewhat morecomplex than those used in prior systems, the increase in complexitydoes not require an unwarranted increase in equipment costs while, atthe same time, the ability of an intruder to fool the system isconsiderably reduced in comparison to present systems.

In this system of the invention, a two-way communication link isutilized between a central monitoring station and one or more remotelocations which are to be protected. Further, the system utilizes abasic signal which is truly random in nature and which is generated atthe monitor station for transmission to a transponder station at aremote location. The transponder appropriately encodes the random signalwith alarm status information using a specifically selected code programnetwork to form specific code words representing the different alarmstatus conditions. The random signal is simultaneously encoded at themonitor station so as to produce a plurality of code words representinga plurality of different anticipated alarm status conditions which maybe present at the remote location, the monitor encoder using the samespecifically selected code program network as that used at thetransponder.

The encoded alarm status signal transmitted from the transponder to themonitor station uses, in a preferred embodiment, the same communicationlink as that used to transmit the random signal from the monitor to thetransponder station, through appropriate time-multiplexing techniques.The encoded signal from the transponder is compared at the monitor witheach of the plurality of encoded alarm status signals generated at themonitor. So long as the encoded word from the remote location matchesthe encoded all clear" word of the monitor with which it is beingcompared, an all clear output signal is generated at the monitor.However, when the encoded word received from the remote location is thesame as that of one of the plurality of encoded alarm signals generatedat the monitor, an alarm output signal is produced to indicate theparticular alarm status involved, such output signal being thereuponused to activate an audible and/or a visual display unit. As used hereinthe term alarm status is used to include an indication of an all clearstatus at the remote location.

Moreover, in the system of the invention, appropriate logic circuitry inthe comparator unit at the monitor is used to detect and display thepresence of a line fault, that is, a fault occurring in thecommunication link itself, e.g., a transmission line which is eitheropened or short-circuited. Further, should any other abnormal conditionoccur, resulting from a malfunction of the equipment at the remotelocation, for example, or from an attempt by an intruder to substitutean incorrect bit stream on to the communication link, such a troublecondition is also appropriately detected and displayed at the monitor.

As mentioned above, the basic random signal is generated at the monitorstation and is transmitted to the remote location for encoding and thenretransmitted in encoded form to the monitor. The fundamentalencoder/decoder design used in the invention permits an extremely largenumber of different codes to be used in the system in conjunction withthe randomly generated signal. Such codes can be changed in a periodic,or non-periodic, manner known only to the user of the system. With sucha large selection of codes available, even were an intruder somehow ablesuccessfully to analyze the signals in the system of the invention forone specific code scheme in order to simulate a counterfeit all-clearsignal, a difficult enough task in itself, the fact that a user canreadily change the code program greatly reduces the chances that anintruder can successfully insert the simulated signal without detection.Thus, the ability of an intruder to defeat the system becomes in apractical sense effectively negligible.

The detailed operation and configuration of the system of the inventioncan be described best with the help of the accompanying drawings,wherein:

FIG. 1 shows a block diagram of the overall system of a preferredembodiment of the invention;

FIG. 2 shows a more detailed block diagram of the system of FIG. 1;

FIG. 3 shows a more detailed block diagram of the random signalgenerator of FIGS. 1 and 2;

FIG. 3A shows a more detailed circuit and block diagram of the generatorof F IG. 3;

FIG. 4 shows a more detailed block diagram of the monitor transmitterand receiver of FIGS. 1 and 2;

FIG. 4A shows a more detailed circuit and block diagram of thetransmitter and receiver of FIG. 4;

FIG. 5 shows a more detailed block diagram of the monitor encoder ofFIGS. 1 and 2;

FIG. 6 shows a chart illustrating the operation of a circuitconfiguration of FIG. 6A;

FIG. 6A shows a typical circuit configuration of a portion of theencoder of FIG. 5;

FIG. 7 shows a more detailed diagram of one form of the code controlprogram unit of FIG. 5;

FIG. 8 shows a more detailed block diagram of a portion of the monitorsystem of FIGS. 1 and 2;

FIG. 9 shows a more detailed block diagram of the alarm and displayunits of FIGs. 1 and 2;

FIG. 9A depicts certain waveforms to illustrate the operation of theunits of FIG. 9;

FIG. 9B shows a more detailed block diagram of the units of FIG. 9;

FIG. 10 shows a more detailed block diagram of the transponder of FIGS.1 and 2;

FIG. 11 shows a more detailed block and circuit diagram of thetransponder receiver of FIG. 10;

FIG. 11A depicts certain waveforms to illustrate the operation of thereceiver of FIG. 11;

FIG. 12 shows a block diagram of the timing circuitry of FIG. 10;

FIG. 12A depicts certain waveforms to illustrate the operation of thetiming circuitry of FIG. 12;

FIG. 13 shows a more detailed block diagram of a portion of the timingcircuitry of FIG. 12;

FIG. 13A depicts certain waveforms to illustrate the operating of thecircuitry of FIG. 13;

FIG. 14 shows a more detailed block diagram of the transponder encoderof FIG. 10;

FIG. 15 shows a more detailed block diagram of the transpondertransmitter of FIG. 10; and

FIG. 15A shows a more detailed block and circuit diagram of thetransmitter of FIG. 15.

FIG. 1 shows a simplified block diagram of the overall system of theinvention wherein a centrally located monitor station 10 includes arandom signal generator 11 which provides an output signal in digitalform, such signal having a truly random nature, i.e., a signal havingaperiodic characteristics. The random signal is fed to an appropriatetransmitter 12 where it is supplied to a transmission link 13, which ina preferred embodiment may be a two-wire transmission line, for example.The output of random signal generator 11 is also simultaneously fed toan appropriate monitor encoder 14 which operates in accordance with aspecifically selected code program 15 to produce a plurality of encodedsignals, each one of which represents a different alarm status conditionwhich may exist at a remote location which is being monitored.

The remote location is identified in FIG. 1 as including a transponderstation 16 which utilizes an appropriate data receiver 17 for receivingthe random signal transmitted by the monitor station from transmissionlink 13. The received signal is fed to a suitable transponder encoder 18which functions in accordance with code program 19. Encoder 18 isadapted to be responsive to one or more different alarm input signalsreceived from one or more sensors (not shown), one of which is activatedin accordance with the alarm status of the remote location. Accordingly,the encoder 18 produces an encoded signal which represents theparticular alarm status of the remote location, the encoded signal beingthereupon transmitted back to monitor 10 via data transmitter 21 andtransmission link 13.

At monitor the encoded alarm signal transmitted from transponder 16 isreceived by receiver 22 and is thereupon fed to an input of an alarmcomparator/decoder 23 which also has fed to it the encoded signals frommonitor encoder 14. The received signal from receiver 22 is thencompared in sequence with the plurality of encoded signals from encoderl4 and produces an output signal only when the two signals beingcompared represent code words which are identical. The output fromdecoder 23 is connected to a plurality of alarm output and displaysystems 24 which are used to display the alarm status of the removelocation when an output signal indicating a matching of the codedcharacteristics of the input signals to the comparator/decoder occurs.

The operation of each of the above subsystems shown in the simplifiedoverall block diagram of the invention is discussed in more detailbelow.

FIG. 2 shows a more detailed block diagram of the overall system shownin FIG. 1. The monitor 10 is typically located at a central alarmreceiving facility, such as a central privately operated station, apolice station, or the like. In general, the monitor station generates adigital signal of a truly random nature and transmits such signal to theremotely located transponder whose alarm status is to be monitored. Themonitor further provides for the encoding of the digital random signalat the monitor to produce a plurality of encoded signals for comparisonwith the encoded alarm signal received from the transponder.Accordingly, the received alarm signal is compared sequentially to eachof the encoded comparison signals generated at the monitor, the alarmstatus of the remote location being extracted as a result of thecomparison. The monitor further provides for an audible and/or visualannunciation or display of the alarm status.

In FIG. 2 the digital random signal from generator 11, which is fed totransmitter 12 for transmittal to the remote transponder location, issimultaneously stored in a data storage unit 25 for subsequent use inencoder 14 in accordance with a timing signal from a timing circuit 26which operates in response to a suitable clock 27. Timing and datastorage units are required at various positions in the system becausethe processing of the data is performed sequentially and because thetransmitted and received signals are carried on the same two-waytransmission link at different times using known techniques for timedivision multiplexing.

The encoder 14 at monitor 10 utilizes a selected code which isdetermined by code control program unit which is responsive to aplurality of simulated signals each representing a different anticipatedalarm condition (including an All Clear condition) which may arise atthe remote location which is being monitored. Encoder 14 therebyoperates upon the random digital signal stored in data storage unit 25and produces a plurality of encoded signals, each one of whichrepresents a different encoded alarm signal or All Clear signal.

The transponder unit 16 receives the digital random signal transmittedfrom monitor 10 and stores the received signal in data storage unit 28.This received signal also includes appropriate synchronizing informationin the form of a suitable timing pulse for actuating timing circuitry 29in the transponder subsystem l6.

Encoder l8 encodes the signal from data storage unit 28, at anappropriate time, in accordance with code control program unit 19 thecode program of which is selected to be the same as that used for codecontrol program unit 15 in the monitor 10. As discussed more fullybelow, an extremely large number of code programs may be selected foruse in the system of the invention. In each case, the same code controlprogram is selected and used in both the monitor and transponder unitsduring operation at any one time.

One of a plurality of alarm signals which indicates the alarm status atthe remote location where the transponder is positioned is suitablyinserted into the signal which is encoded by encoder 18, so as toproduce a coded alarm status information signal which again isappropriately stored in a data storage unit 31, the operation of whichis suitably timed to produce an encoded alarm information output signalfor transmission via transmitter 21 and transmission link 13 back tomonitor 10.

The receiver unit 22 at monitor 10 then feeds such signal to datastorage unit 32, the operation of which is appropriately timed so as toproduce a signal for feeding to comparator/decoder unit 23 which alsoreceives the signals from encoder unit 14. Decoder unit 23 thereuponcompares the received encoded alarm information signal from data storageunit 32 with each of the plurality of encoded signals from encoder 14 ina sequential manner to determine which of the latter signals has thesame characteristics as the coded alarm information signal fromtransponder 16.

The comparator/decoder 23 is connected to a plurality of alarm displaysubsystems 24. As discussed more fully below, decoder unit 23 producesan output when the signals being compared have matching characteristicsand such output activates one of the alarm display subsystems at a timedepending on the alarm status information contained in the encodedsignal from the transponder 16. In addition the decoder 23 is alsoarranged to produce output signals for indicating an All Clear status, aLine Fault status, or a Trouble status which represents a malfunction inthe system which may have resulted from incorrectly operating, ornon-operating, units of the system or from tampering with the system byan intruder. The operation of all of the above subsystems and unitstherein is described more fully with reference to the remaining figures.

Monitor Random Signal Generator The purpose of random signal generator11 is to produce a random sequence of digital bits (i.e., ones andzeros) which are non-periodic in nature, so that the sequence has trulyrandom characteristics. The random bits are then grouped in series toform random digital groups or words which form the basic message signalfor communication between the monitor and the transponder subsystems.Although not limited thereto, the invention is described herein withreference to the use of 4-bit groups in each digital word fortransmission from the monitor to the transponder and return.

FIG. 3 shows a functional block diagram of one embodiment of randomsignal generator 11 which includes a noisy oscillator 33 which is anoscillator designed to be relatively unstable so as to produce in effecta signal having a frequency f, which is made up of a basic frequency fand has superimposed thereon a relatively rapid frequency jitter ofnoise, represented byiAflt) about the average or basic frequency f,,.The frequency f is designed to be substantially higher than the bit ratefrom the random signal generator 11 and is also statisticallyindependent of the bit rate. The oscillator output (which by appropriatelimiting means provides either a digital one or a digital zerooscillation) is sampled at a bit rate determined by stable clock 34which produces a gating signal comprising bits with a width 7 at afrequency f where f, f and r l/f,. The gating signal is applied to anappropriate gated storage unit 35 which produces a random bit stream atthe output thereof. The output random bit stream has essentially nobit-to-bit correlation and no message-to-message (i.e., bit group to bitgroup) correlation.

FIG. 3A illustrates a specific implementation of the system shown inFIG. 3. In that figure the oscillator comprises three digital inverters,40, 41 and 42 interconnected as shown with a resistor 43 connectedbetween inverters 40 and 41 and a resistor 44 connected between theinput of the inverter 40 and the output of the inverter 42. A capacitor45 is connected in parallel with inverter 40 and resistor 43 as shown.The values of resistors 43 and 44 and capacitor 45 determined the timeconstants of the circuitry and, accordingly, determine the oscillationfrequency. The oscillation, however, is highly unstable in frequency sothat the output signal effectively has an average frequency on which issuperimposed a relatively rapid jitter, or noise, frequency signal, asdiscussed above. Relatively short strobe pulses at the desired bit rate(i.e., l lf are supplied fr om stable clock pulse generator 34 to awell-known JK flip-flop circuit 36, the J and 1 iinput terminals ofwhich are connected to the output of the unstable frequency oscillatorand the timing strobe pulses being connected to input terminal t. The

v flip-flop circuit samples and stores the oscillator output andproduces an output v, therefrom an output terminal O which changes onlywhen the strobe pulse occurs, such output representing the most'recentlysampled oscillator output state. Accordingly, the output of flip-flopcircuit 36 is a sequence, or stream, of bits (i.e., ones" or zeros)which are truly randomly sequenced. Monitor Line Transmitter andReceiver A block diagram of the monitor transmitter receiver circuitryis shown in FIG. 4, the transmitter 12 (shown by dashed line 50) beingin the form of a switchable current source connected amplifier having apower supply 51 producing a voltage V,, as the power source therefor.Transmission of data bits on transmission link 13, which in a preferredembodiment is a two-wire transmission line, is achieved by the presenceor absence of current flow I, in the transmission line. The current flowis controlled by the state of the logic input signal v, which isreceived from random signal generator l l, where v, a v, 0v) or a one"(v, +v). THe magnitude of the current flow is controlled by a referencevoltage v from a suitable reference source 52. The logic input signal v,and reference voltage signal v are fed to operational amplifier 53, theoutput of which is fed to a power amplifier 54 with a current feed backloop 55 producing a current dependent feed back voltage across aresistor 56, as shown. A specific circuit configuration for transmitter12 is shown in FIG. 4A wherein the logic input voltage v, representingthe random data to be transmitted is fed through a diode 57, to oneinput of amplifier 53 via input resistor 58, the voltage reference vbeing fed to the other input thereof. The output of amplifier 53 is fedvia diode 59 to the base of a transistor across resister 61. Currentfeed back is supplied from the transistor 60 to the input of amplifier53 across resistor 56.

If the logic input voltage v, is high (+5v), diode 57 conducts and theoutput of amplifier 53 is negative so that diode 59 is non-conductingand the base of transistor 60 is at ground potential. The transistor isthereby cut off with no collector current thereby inhibiting the flow oftransmission line current.

If v, is low (0v.), diode 57 is off and the output of amplifier 53 risesso that diode 59 conducts and transistor 60 is turned on, therebyallowing the flow of transmission line current 1,. Current I, flowsthrough resistor 56 to produce a feed back voltage v proportional to 1,,i.e., V =1, R The feed back control is such that I, V,,,/R, v /R Afeature of the transmission line signaling system as shown in the FIGS.4 and 4A is that the signal current will remain approximately constant(I, v /R regardless of the transmission line length up to some maximumlength. The transmission line, for example, offers a resistance R whichis proportional to its length. The maximum transmission line resistanceR,,, max for a given signal current I, and power supply source voltage VL is approximately V /I, While 59 is not essential to the operation ofthe circuit shown in FIG. 4A, its use prevents the reverse base-emittervoltage breakdown of transistor 60 from being exceeded and also protectsamplifier 53 if the collector-base voltage breakdown of transistor 60 isexceeded as, for example, because of a spike of voltage noise on thetransmission line.

THe monitor receiver circuitry 22 is also shown in FIGS. 4 and 4A asenclosed in dashed line and includes a low pass filter circuit 61 havinga received voltage signal, v,., supplied thereto across resistor 62, theoutput of the filter being supplied to one end of a voltage comparatorcircuit 63 via resistor 64. The other input of voltage comparator 63 isa voltage reference v' from a suitable reference source 66. Tile signalto be detected is the presence of absence of current on the transmissionline 13 as received from transponder l6. Resistor 62 is a currentsensing resistor having a value R, in series with the transmission lineso that v, I, R, When line current is flowing, v, is greater than v' andthe comparator data output is a logic zero" signal. When no line currentis flowing, v' is greater than v,, the latter being approximatelyequalto zero, and the comparator data output is a logic one signal. Inthis way, the on-off signal current between the monitor and thetransponder is converted to appropriate logic level signals by themonitor line receiver. The low pass filter 61 prevents noise spikes andother highfrequency noise typically encountered on long transmissionlines, such as long wire lines, from causing false data to be suppliedat the output of comparator 63.

Encoder

1. An alarm system for monitoring at a first station the alarm status ofat least one second station, said system comprising a monitor located atsaid first station, said monitor including means for generating a randomsignal; means for producing one or more simulated signals correspondingto one or more alarm status conditions at said second station; monitorencoding means responsive to said random signal and to said one or moresimulated signals for producinG one or more encoded monitor signals,said latter signals being encoded with a selected code program; monitortransmitter means for feeding said random signal to a transmission linkmeans for transmission to said second station; a transponder located atsaid second station, said transponder including means for receiving saidtransmitted random signal; transponder encoder means responsive to saidreceived random signal and adapted to be responsive to a signalrepresenting the alarm status of said second station for modifying saidreceived random signal to produce an encoded transponder signal, saidlatter signal being encoded in accordance with said selected codeprogram; transponder transmitter means for feeding said encodedtransponder signal to said transmission link means for transmission ofsaid signal to said first station; said monitor further including meansfor receiving said transmitted encoded transponder signal; means forcomparing said encoded transponder signal with each of said encodedmonitor signals to produce an alarm output signal when thecharacteristics of said encoded transponder signal are the same as thecharacteristics of at least one of said encoded monitor signals; anddisplay means responsive to said alarm output signal for annunciatingthe alarm status of said second station.
 2. An alarm system inaccordance with claim 1 further including timing means for timing theoperation of said random signal generating means, said monitor encodingmeans, said transponder encoding means, said comparing means, and saiddisplay means whereby said monitor encoding means produces said one ormore encoded monitor signals and said transponder encoding meansproduces said encoded transponder signal in response to the same randomsignal generated from said random signal generating means for comparisonat said comparing means.
 3. An alarm system in accordance with claim 2wherein said monitor encoding means includes first data storage meansresponsive to said random signal for feeding said random signal data toan encoder in parallel form; second data storage means responsive tosaid encoded transponder signal for feeding said encoded transpondersignal data to said comparing means in parallel form; and said timingmeans includes means for timing the operation of said first and saidsecond data storage means.
 4. An alarm system in accordance with claim 2wherein said transponder encoding means includes first data storagemeans responsive to said transmitted random signal for feeding saidrandom signal data to an encoder in parallel form; second data storagemeans responsive to said encoded transponder signal for feeding saidencoded transponder signal to said transponder transmitter means inseries form; and said transponder timing means controlling the operationof said first and said second transponder data storage means.
 5. Analarm system in accordance with claim 1 wherein said display meansincludes means for audibly displaying said alarm status.
 6. An alarmsystem in accordance with claim 1 wherein said display means includesmeans for visually displaying said alarm status.
 7. An alarm system inaccordance with claim 1 wherein said random signal generating meansincludes an unstable oscillator for producing a plurality of pulses at arandomly varying frequency; gated storage means for receiving saidplurality of pulses; and clock means for producing a series of gatingpulses at a fixed frequency for actuating said gated storage means toproduce an output stream of random bits.
 8. An alarm system inaccordance with claim 7 wherein said randomly varying frequency is f1and is of the form f0 + or -f(t), said fixed frequency is f2, and thewidth of said gating pulses is Tau , the frequency f1 being much greaterthan the frequency f2 and said pulse width Tau is much less than 1/f1.9. An alarm system in accordance with claim 1 wherein said monitortransmitter means includes means for providing a reference voltage;voltage comparator means responsive to said reference voltage and to therandom signal from said random signal generator; means responsive to theoutput of said voltage comparator means and connected at its output tosaid transmission link for producing a controlled current pulse on saidtransmission link when the value of said random signal exceeds that ofsaid reference voltage and for producing a zero current on saidtransmission link when said random signal value is less than that ofsaid reference voltage; and power supply means for supplying power fortransmitting said controlled current pulse on said transmission link.10. An alarm system in accordance with claim 9 wherein said transmissionlink is a transmission line; and said current pulse is producedindependently of the length of said transmission line.
 11. An alarmsystem in accordance with claim 1 wherein said monitor receiving meansincludes means responsive to received current pulses on saidtransmission link for producing a voltage pulse; filter means responsiveto said voltage pulse for producing a filtered voltage pulse; means forproviding a reference voltage; voltage comparator means responsive tosaid filtered voltage pulse and to said reference voltage for producingan output pulse representing the received signal data on saidtransmission link.
 12. An alarm system in accordance with claim 3wherein said first data storage means is a shift register for storingsaid random signal input data in serial form and for providing saidsignal data at its output in parallel form; a plurality of logicnetworks responsive to parallel signal data for producing encodedparallel signal data; a code control program network interconnecting theinputs of said logic networks with said output signal data from saidshift register and with simulated alarm status information signals inaccordance with a selected interconnection program.
 13. An alarm systemin accordance with claim 12 wherein said logic networks include aplurality of exclusive/or gates.
 14. An alarm system in accordance withclaim 13 wherein each such logic network includes a first exclusive/orgate having two inputs connected to said code control program networkand an output connected to said code control program network; a secondexclusive/or gate having one input connected to the output of said firstexclusive/or gate and a second input connected to said code controlprogram network, the output of said second exclusive/or gate therebyproviding an encoded data bit.
 15. An alarm system in accordance withclaim 1 wherein said display means includes a plurality of displayunits, each being responsive to a different one of said alarm outputsignals and including a plurality of storage means adapted to store saidalarm output signals; means for feeding said alarm output signalssequentially to each of said plurality of storage means.
 16. An alarmsystem in accordance with claim 15 wherein said sequential feeding meansincludes means for generating strobe pulses; gating means for feedingsaid strobe pulse to said plurality of storage means for sequentialoperation of said storage means; and means for actuating said gatingmeans.
 17. An alarm system in accordance with claim 16 wherein saidgating actuating means includes detector means responsive to saidencoded monitor signal for producing an output signal if all the databits in said encoded monitor signal are in either a zero or a one state,said output signal preventing the actuation of said gating means whenpresent and permitting the actuation of said gating means when absent.18. An alarm system in accordance with claim 15 and further includingalarm gate means connected to the outputs of said storage means forproducing a gating output signal if any of said storage means has analarm output signal stored therein and for producing a zero signal ifnone of said alarm storage means has an alarm output signal storedtherein; detector means responsive to said encoded transponder signalfor producing an output signal if all of the data bits in said encodedtransponder signal are in either a zero or a one state; logic circuitmeans responsive to said further gate means and to said detector meansfor producing a first output signal when an output signal is presentfrom said detector means and no output signal is present from saidfurther gate means for producing a second output signal when no outputsignal is present from said detector means and no output signal ispresent from said or gate means; first means for displaying the presenceof said first output signal; and second display means for displaying thepresence of said second output signal.
 19. An alarm system in accordancewith claim 15 wherein each of said plurality of display units furtherincludes averaging means for producing an averaged voltage in responseto a stored alarm output signal; threshold reference means for producinga threshold reference voltage; voltage comparator means responsive tosaid averaged voltage and to said threshold reference voltage forproducing an output voltage when said averaged voltage is greater thansaid threshold voltage; and latching circuit means responsive to theoutput voltage from said voltage comparator means for actuating adisplay device for displaying the presence of an alarm output signal.20. An alarm system in accordance with claim 19 wherein said displaydevice includes a visual display unit; a source for producing an on-offvoltage; an and gate responsive to the output of said latching circuitand to said on-off voltage for producing a display output signal when avoltage is present from said latching circuit and said on-off voltage isin the on state, said visual display unit being responsive to saiddisplay output signal from said ''''and'''' gate to produce a blinkingvisual effect.
 21. An alarm system in accordance with claim 4 whereinsaid transponder receiving means includes a signal filter meansresponsive to said received random signal to provide a filtered receivedsignal; detector means for detecting the presence and absence of acurrent pulse in said filtered signal and for feeding said detectedpulses to said timing means and to said first transponder data storagemeans.
 22. An alarm system in accordance with claim 21 wherein saidtiming means includes a timing chain generator for producing a pluralityof timing pulses for operating said first and said second transponderdata storage means and said transponder encoder; means for producingclock pulses for the actuation of said timing chain generator;synchronous detector means responsive to said detected pulses and to atiming pulse from said timing chain generator for producing a resetsignal to reset said timing chain generator to its initial condition atthe end of each message frame of said received signal; said timing chaingenerator producing a strobe output pulse at a selected time in saidmessage frame for stopping the operation of said clock pulse producingmeans.
 23. An alarm system in accordance with claim 22 wherein saidsynchronous detector includes a monostable multivibrator means forproducing said reset pulse in response to said strobe pulse from saidtiming chain generator; and trigger circuit means responsive to saidreceived detected pulses for triggering the operation of said monostablemultivibrator.
 24. An alarm system in accordance with claim 21 whereinsaid transponder receiving means further includes means connected tosaid transmission link means for sensing the current in saidtransmission link means, said current sensing means being connected tothe input of said filter means; and said detector means includesreference voltage source means; and voltage comparator means having afirst input connected to said reference voltage source means and asecond input connected to the output of said filter means, said voltagecomparator means producing said detected pulses, said pulses includingsaid random signal transmitted from said monitor transmitter means and asynchronous pulse for actuating the timing means of said transponder.25. An alarm system in accordance with claim 1 wherein said transpondertransmitter means includes switching means responsive to said encodedtransponder signal for interrupting a continuous current signal on saidtransmission link means in response to the encoded data of saidtransponder encoded signal.